1:45 PM - 2:00 PM
[SMP37-12] In-situ boron isotope study of forearc srpentinites from the Franciscan Complex, California
Keywords:serpentine, forearc serpentinite, boron isotope, spot analysis, Franciscan Complex, LA-MC-ICPMS
Boron concentration and isotpe composition of serpenines in the investigated serpentinites show a large variation (B=3-239 µg/g, δ11B=-12.0 to +24.4 ‰). Serpentinites with blueschist-facies high-pressure metamorphic rocks is characterized by lighter boron isotope, with negative δ11B values. In contrast, massive serpentinites without high-pressure metamorphic rocks show heavier boron isotope, with positive δ11B values. Considering the geological context and the fact that serpentinites with higher δ11B values are enriched in boron, the isotope trend in forearc serpentinites can be explained essentially by fluid evolution via a Rayleigh fractionation during progressive dehydration of subducting slab. However, detailed evaluations in Edgewood Park with the largest isotope variation have led to an interpretation of multi- source of fluids that have facilitated serpentinization at different environments. Moreover, a mineralogical assessment on relict mantle minerals in serpentinites found a possibility that the δ11B values correlate with the degree of partial melting of original peridotite before the serpentinization.